Typical encapsulants for LED devices are organic polymeric materials. Encapsulant lifetime is a significant hurdle holding back improved performance of high brightness LED devices. Conventional LED devices utilize epoxy resin encapsulants and, when is use, epoxy resins often yellow over time. This can reduce brightness and change the color rendering index of light emitted by the device. This is particularly important for white LED devices. The yellowing of the epoxy is believed to result from decomposition induced by high operating temperatures of the device and/or absorption of UV-blue light emitted therefrom.
A second problem that can occur when using conventional epoxy resins is stress-induced breakage of the wire bond on repeated thermal cycling. High brightness LED devices can have heat loads on the order of 100 Watts per square centimeter. Since the coefficients of thermal expansion of epoxy resins typically used as encapsulants are significantly larger than those of the semiconductor layers and the moduli of the epoxies can be high, the embedded wire bond can be stressed to the point of failure on repeated heating and cooling cycles.
Thus, there is a need for new photochemically stable and thermally stable encapsulants for LED devices that reduce the stress on the wire bond over many temperature cycles. In addition, there is a need for encapsulants that can be cured rapidly in order to accelerate manufacturing times and reduce overall LED cost.